1. Technical Field of the Invention
The present invention relates to a switching power supply apparatus which intermittently supplies the power from an input power supply, converts the power by an inductor, and outputs a predetermined DC voltage.
2. Description of the Related Art
Generally, a switching power supply apparatus has a harmonic property and a power-factor property. The harmonic property corresponds to a suppressing function of harmonic current flowing to an input power line from the switching power supply apparatus, and has a predetermined upper limit of the harmonic current to prevent any harmful influences on another device. The power-factor property corresponds to a power factor of input of the switching power supply apparatus and, preferably, may be increased so as to reduce the loss of a power system.
Japanese Unexamined Patent Application Publication No. 11-187664 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 4-21358 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 7-75334 (Patent Document 3) disclose conventional switching power supply apparatuses.
FIG. 14 shows the structure of the switching power: supply apparatus disclosed in Patent Document 1. Referring to FIG. 14, a first switching circuit S1 is a parallel circuit including a first switching element Q1, a first diode D1, and a first capacitor C1. A second switching circuit S2 is a parallel circuit including a second switching element Q2, a second diode D2, and a second capacitor C2.
Reference numeral T denotes a transformer, and a first switching circuit S1 and an input power supply E are serially connected to a serial circuit including a primary winding T1 and an inductor L. A serial circuit including a second switching circuit S2 and a capacitor C is connected to a serial circuit including the primary winding T1 and the inductor L in parallel therewith. A secondary winding T2 of the transformer T includes a rectifying and smoothing circuit RS including a rectifying diode Ds and a smoothing capacitor Co. A capacitor Cs is connected to the rectifying diode Ds on the secondary side in parallel therewith. A detecting circuit 14 detects an output voltage Vo supplied to the load and output current Io, if necessary. A control circuit 11 inputs a voltage generated by a bias winding T3, and provides positive feedback to the switching element Q1, thereby self-oscillating the switching element Q1. A control circuit 12 inputs a voltage generated by a bias winding T4 and controls the off-timing of the switching element Q2, thereby controlling the on-period of the switching element Q2.
FIG. 15 shows the structure of the switching power supply apparatus disclosed in Patent Document 2. Referring to FIG. 15, an AC voltage supplied from an AC power supply 2 is rectified by a rectifier 4, and is smoothed by a smoothing capacitor 6, thereby obtaining a rectifying voltage Vin. Then, the rectifying voltage Vin is supplied to a first power converting unit 8 and a second power converting unit 10. When the switching transistor Qs is turned-on, the rectifying voltage Vin is added to a primary winding L1 including a choke coil CH, a diode Db, and a high-frequency transformer T, thereby charging the energy to the choke coil CH. The switching transistor Qs is turned-off, the energy of the choke coil CH sends the current to the diode Dc, the primary winding L1, and the capacitor C1. The on/off-operation of the switching transistor Qs is repeated and the voltage induced to a secondary winding L2 of the transformer T is smoothed by a diode D2 and a capacitor Co, thereby outputting a DC voltage Vo. A pulse width control circuit 16 controls the energization time of the switching transistor Qs in accordance with the change in output voltage Vo, thereby stabilizing the output voltage Vo.
FIG. 16 shows the structure of the switching power supply apparatus disclosed in Patent Document 3. Referring to FIG. 16, a full-wave rectifying circuit 2 inputs an AC input voltage from an input terminal 1-1′, thereby outputting a rectifying voltage Ei. A first capacitor 3 smoothes the current of an inductor 20 via a second switching element 6 and a second capacitor 7, and supplies a DC voltage E3. A first switching element 4 converts, into an AC voltage, the rectifying voltage Ei via the inductor 20 and the DC voltage E3 of the first capacitor 7 via a primary winding 51 of a transformer 5, via a high-frequency switching operation. The second switching element 6 and the first switching element 4 are alternately turned on/off by the control circuit 11. The second capacitor 7 absorbs and releases a portion of excitation energy stored in the transformer 5 and the current of the inductor 20 for an on-period of the second switching element 6. A diode 8 and a capacitor 9 define a rectifying and smoothing circuit. The rectifying and smoothing circuit rectifies and smoothes a fly-back voltage of an AC voltage with a high frequency generated to a secondary winding 52, and further outputs a DC output voltage Eo to an output terminal 10-10′. The control circuit 11 detects the DC output voltage Eo and controls an on/off ratio of the first switching element 4 and the second switching element 6.
However, as disclosed in Patent Document 1, a voltage clamping circuit performs a zero-voltage switching operation (hereinafter, referred to as ZVS operation), thereby improving the efficiency. However, a harmonic-current suppressing function is not provided.
As disclosed in Patent Document 2, the harmonic-current suppressing function is provided. However, the ZVS operation is not executed. Therefore, the loss of switching operation is large and the circuit efficiency deteriorates. When the power supply is shut-off from the input power supply at the prompt stop of power supply, the voltage is greatly increased at the low load and is above the withstand voltage of the elements because of a non-control operation of a voltage of the capacitor C1 for ensuring an output holding time for continuously supplying the output for a predetermined time period.
As disclosed in Patent Document 3, a voltage clamping circuit performs the ZVS operation, and includes the harmonic-current suppressing function. However, the current generated by the switching operation flows to a diode for rectifying a commercial AC voltage (corresponding to the full-wave rectifying circuit 2 shown in FIG. 16), and therefore, the loss of the diode is substantial and the reducing effect of the harmonic current is reduced. Therefore, a low-pass filter must be provided in the commercial AC power line. Similar to Patent Document 2, since the voltage of the capacitor 3 for ensuring that the output holding time is not controlled, there is a problem in that the voltage greatly increases at the low load and the voltage exceeds the withstand pressure of the components.